The present invention relates to a device and process using a laser beam to scan an area of an object, in particular for selective laser melting a metallic powder for fabricating a mold, for example, a prototype of a component.
The present invention relates predominantly to a technology referred to as rapid prototyping. Rapid prototyping processes are employed in product development to shorten product development time and increase product quality, which is made possible by the fact that with rapid prototyping processes prototypes can be quickly produced directly from the 3D CAD model, thereby obviating the hitherto required time-consuming creation of a NC program for milling, eroding or fabrication of form-giving tools.
The object of the development of new, respectively further development of present, rapid prototyping processes is to be able to process materials that are as close as possible to the series material or even identical to it. This especially applies to metallic prototypes or prototype tools. The known processes for selective laser melting permit fabricating components from commercial steels. These components are fabricated, as in all rapid prototyping processes, in layers. For this purpose, the material is applied in powder form as a thin layer onto a building platform. The powder is locally melted using a laser beam according to the configuration of the component of the to-be-processed layer. Steel components (e.g. stainless steel 1.4404) fabricated with this process attain, with regard to density and strength, the prescribed material specification. Thus, they can be used as function prototypes or directly as finished components.
DE 196 49 865 C1 proposes a process in which a metallic powder of a material containing no binding agent and no fluxing agent is applied to the building platform and is heated to the melting temperature by the laser beam according to the configuration of the component. The laser beam energy is selected in such a manner that the entire thickness of the layer of metallic powder of the material is completely melted where the laser beam impinges. In this process, the laser beam is led in several paths over the preset area of the respective layer of powder material in such a manner that each successive laser beam path partially overlaps the previous beam path. At the same time, a protective gas atmosphere is maintained over the interaction zone of the laser beam in order to prevent faults, which might be caused, for instance, by oxidation.
In the selective laser melting process, the focused laser beam scans the area of each layer belonging to the contour of the component line by line. The obtainable detail resolution and the surface quality of the produced parts depends decisively on the diameter of the focused laser beam.
Hitherto two fundamentally different technologies are known for moving a focused laser beam on a stationary processing plane.
Usually optical scanner systems are employed in rapid prototyping using a laser beam respectively a light beam. In the scanner system, positioning and moving the focused laser beam on the processing plane for each direction (x- and y-direction) occurs by turning a mirror respectively. The use of a scanner optic in a process for laser-beam sintering is, for example, schematically illustrated in the publication by Heinz Haferkamp et. al., Laserstrahl-Sintern zur Herstellung von Blechformwerkzeugen, in the journal xe2x80x9cBLECH ROHRE PROFILExe2x80x9d, 43 (1996) 6, pp. 317-319.
The disadvantage of scanner systems, however, is that if the preset turning angle of the mirror is maximum, the size of the workable area depends on the focal length of the focusing optic. The workable area can only be enlarged by means of a longer focal length. However, if all other optical elements are identical, increasing the focal length will also increase the focal diameter of the laser beam, thereby reducing the obtainable detail resolution and the quality of the surface of the produced parts.
From other areas of material processing with lasers, it is also known to use plotter systems. In a plotter system, the laser beam is guided along two linear axes. By suited movement of the axes, the laser beam can describe any path on the processing plane.
Using a plotter system has the advantage that the size of the workable area is only limited by the length of the employed linear axes. Moreover, a protective gas nozzle can be simultaneously moved along with the laser beam in a simple manner by coupling it to the linear axes.
However, the disadvantage of the plotter system is that, due to the mechanics involved, in comparison to a scanner system, only substantially lower processing velocities can be realized with a plotter system. In particular, processing velocities of  greater than 200 mm/s preferred for selective laser melting cannot be realized with sufficient accuracy with a plotter system.
A process for fabricating elongated components with diameters in the micro-range by means of laser melting, in which a stencil with elongated indentations according to the shape of the micro-parts, into which the metallic powder is filled, is employed is known from DE 34 45 613 C1. The laser beam is focused through the window of a high-vacuum chamber onto these indentations and the powder located therein is melted by a slow movement of the laser beam along the indentations. Subsequently, the melted material solidifies to a finished component. The respective device is provided with a scanner optic which can travel along a carrier arm parallel to the surface of the object. The purpose of a further linear axis is to set the distance of the scanner optic to the surface.
However, this printed publication does not relate to the field of so-called rapid prototyping but rather to the direct production of metallic micro-parts by means of a stencil employing very low processing velocities between 0.1 mm and 100 mm an hour.
The object of the present invention is to provide a device and a process using a laser beam to scan an area of an object, in particular, for selective laser melting permitting a sufficiently large processing area and a high processing velocity with a small focused laser beam diameter.
The object of the present invention is solved with the device according to claim 1 and the process according to claim 10. Advantageous embodiments of this device and this process are the subject matter of the subclaims.
A key element of the present invention is that, in selective laser melting or laser sintering, when impinging on the processing area, the diameter of the laser beam is smaller than the width of the to-be-melted area respectively to-be-melted structure. Consequently, it is necessary to guide the laser on several adjacent paths to cover the entire to-be-melted area.
An element of the present invention is that it was understood that, by suited division of this area respectively of the laser paths on the area, two varyingly fast systems can be used to guide the laser beam. While the laser beam can be moved back and forth with high velocity to scan small subsections by means of a scanner optic, such as is known from the state of the art, the entire scanner optic is moved further over the processing area with the aid of two linear axes to reach further subsections. Only a low velocity is required for this linear movement, which occurs largely transverse to the scanning direction of the scanner optic. The scanning movement of the scanner optic itself has to cover only a small area so that a focusing optic with a short focal length can be employed. Thus, the invented device provides a plotter system with two independent linear axes respectively linear drives on which, in addition, a scanner optic is provided.
The invented device, therefore, combines the advantages of the rapid movement of a focused laser beam by means of a scanner with the advantages of the size of the processing field being independent of the focal length of the focusing optic as a result of using a plotter to move the beam. Furthermore, this device permits, in a simple manner, guiding a protective gas nozzle along with it. The laser beam is preferably focused with a short-focal-length optic to a diameter of  less than 200 xcexcm and therefore simultaneously an area of any size can be worked with velocities of  greater than 200 mm/s.
In the invented process, the to-be-processed area is preferably divided into strip-shaped subsections (strips). Processing of the whole area by processing the individual occurs consecutively. The individual strips are processed by the laser beam being guided over the area inside a strip path by path, requiring a high laser beam velocity inside a path. The velocity with which the laser beam is moved from path to path is comparably lower.
A scanner is mounted on a plotter for moving the laser beam. The rapid movement of the laser beam inside a path is executed by the scanner. The movement of the laser beam from path to path and from strip to strip is executed by the plotter with the entire scanner being moved further along with the plotter.
As with this arrangement, the deflection of the laser beam by the scanner only occurs inside the respective strip, the maximum excursion of the laser beam by the scanner corresponds to the width of the strip. If a small strip width is selected (e.g.  less than 5 mm), laser radiation can be focused with a short focal-length-focus optic (e.g. f less than 100 mm) onto a correspondingly small diameter (e.g.  less than 200 xcexcm). Suited selection of the strip width and the focusing optic also permits obtaining laser-beam-focal diameters of 10 xcexcm) in the processing plane.
By mounting a protective gas nozzle on the plotter, the protective gas nozzle is moved along with the plotter motion. However, as the nozzle is only moved along with the plotter but does not follow the laser movement by means of the scanner, the width of the nozzle aperture is selected larger or the same size as the width of the strip, thereby ensuring that there is a continuous flow of protective gas in the interaction zone of the laser beam.